Modern diesel engines typically use a common rail fuel system that employs a fuel pump to supply diesel fuel at injection pressure to a plurality of fuel injection valves for injecting the fuel directly into each cylinder. That is, common rail fuel injection valves do not intensify fuel pressure inside the fuel injection valve. An advantage of common rail fuel systems is that fuel pressure and fuel injection events can be controlled independently of engine speed.
Known common rail injection valves are hydraulically actuated, whereby a control chamber filled with hydraulic fluid is used to control valve actuation. Generally, hydraulic fluid is held at pressure in the control chamber to hold the valve in an open or closed position. For example, the pressure of the hydraulic fluid in the control chamber can be employed to hold the valve needle in the closed position, and to open the fuel injection valve a control valve can be operated to drain the hydraulic fluid from the control chamber, allowing the fuel injection valve needle to move to an open position so a fuel injection event can occur. When the control valve is closed, the hydraulic pressure in the control chamber rises and causes the injection valve needle to move to the closed position to end the fuel injection event.
The control valve typically uses a solenoid actuator, but piezoelectric actuators can also be employed, as disclosed by U.S. Pat. Nos. 6,062,533 and 5,779,149.
A disadvantage of hydraulically actuated fuel injection valves is that it is not possible to control the operation of the control valve to move and hold the fuel injection valve needle to an intermediate position between the closed position and the fully open position. Such fuel injection valves use means other than controlling needle lift to influence the fuel mass flow rate into the engine cylinders. For example, some hydraulically actuated fuel injection valves employ spill ports to drain fuel from the nozzle fuel cavity to influence the fuel mass flow rate into the combustion chamber.
Recent developments have been directed at using the mechanical strain produced by piezoelectric, magnetostrictive, or electrostrictive transducers to provide the mechanical movement to directly actuate the position of the injection valve needle. Fuel injection valves of this type are referred to herein as “directly actuated fuel injection valves”. Examples of directly actuated fuel injection valves are disclosed in co-owned U.S. Pat. Nos. 6,298,829, 6,564,777, 6,575,138 and 6,584,958. In these injection valves, a strain-type transducer can be activated to convert energy from one form to produce a mechanical strain. That is, a piezoelectric, magnetostrictive, or electrostrictive transducer can be activated to produce a mechanical strain that correlates directly to a corresponding movement of the valve needle.
Some advantages of directly actuated fuel injection valves over hydraulically actuated fuel injection valves include: (a) the high force that strain-type transducers can generate; (b) the substantially instantaneous displacement upon activation; and (c) the flexibility of being able to modulate the mechanical strain by simply modulating the energy applied to the actuator, allowing the valve needle to move to, or be held in, any intermediate position between the closed and fully open positions. For example, if the actuator is a piezoelectric actuator, the amount of mechanical strain can be determined by controlling the charge applied to the piezoelectric material. If the actuator is a magnetostrictive actuator, the amount of mechanical strain can be determined by controlling the strength of the magnetic field applied to the magnetostrictive material. Accordingly, with directly actuated fuel injection valves it is possible to use the actuator to manipulate needle position to control the mass flow rate through the fuel injection valve, allowing more flexibility to control the combustion process to improve combustion efficiency and/or reduce engine emissions.
With piezoelectric, magnetostrictive, and other strain-type actuators, the stroke is generally much smaller than the stroke that can be provided by hydraulic or solenoid actuators. Accordingly, it is known to employ a means for amplifying the stroke of a strain-type actuator. For example, U.S. Pat. No. 5,630,550 discloses a directly actuated fuel injection valve that employs a hydraulic displacement amplifier. The '550 patent discloses an arrangement that fluidly isolates the fluid in the hydraulic displacement amplifier from the fuel, but the fuel pressure is not employed to pressurize the hydraulic fluid. A disadvantage with this arrangement is that there is a greater likelihood of cavitation in the hydraulic displacement amplifier if the movement of the actuator causes the fluid pressure to drop below the fluid's vapor pressure. Another disadvantage is an increased likelihood of leakage into or out from the hydraulic displacement amplifier if there are significant differential pressures across the seals that seal the hydraulic displacement amplifier.
U.S. Pat. No. 4,909,440 discloses a fuel injection valve that comprises a piston actuated by a piezoelectric element. The piston operates on a control chamber that is filled with fuel. A rear face of the piston is subjected to high-pressure fuel to cancel out the forces acting on the piston from fuel pressure residing within the control chamber. The '440 patent claims that this arrangement allows more precise control of the opening and closing of the needle because the piston's movement is not influenced by variations in fuel pressure.
U.S. Pat. Nos. 5,697,554 and 4,813,601 (FIG. 3) disclose directly actuated fuel injection valves that employ piezoelectric actuators and hydraulic displacement amplifiers. The arrangements disclosed by these patents have their fuel cavities fluidly isolated from the hydraulic displacement amplifier system, which includes an amplifier chamber, which, in each case is in restricted fluid communication with a low pressure reservoir via “restrictor gaps”. The '601 patent also includes a complicated check valve arrangement for replenishing the hydraulic fluid in the amplifier chamber. In both cases the fuel pressure is not used to influence the fluid pressure in the hydraulic displacement amplifier.
The '554 patent discloses an arrangement whereby hydraulic fluid can flow between the hydraulic displacement amplifier and the low pressure fluid reservoir to compensate for the effects of differential thermal expansion within the fuel injection valve. This feature that can be advantageously incorporated into hydraulic amplification systems. However, thermal and wear effects may not be completely compensated for by such systems and the amplified movement of the injection valve needle may not track the movements of the actuator accurately. To control the movement of a valve needle to follow a predetermined waveform to thereby control fuel mass flow rate and the combustion characteristics, an accurate method of compensating for the thermal and wear effects is needed.